[0001] The invention relates to a method for protecting the guide tube members of a nuclear
fuel assembly according to the precharacterising part of claim 1. A nuclear fuel assembly
for which this method can be used is known, for example, from FR-B-8 211 575 (publication
No. 2 529 704).
[0002] In the fuel assembly disclosed in the afore-mentioned publication the slitted end
portions of the guide members are very vulnerable. By the slitting of said end portion
a plurality of axially directed flaps are formed, which can easily be broken when
the slitted portion is not in its place in the top tie plate, that is, during removal
and reinstallation of the top tie plate, and at intermediate stages.
[0003] The invention aims at developing a method of the above-mentioned kind which provides
good protection for the axially directed flaps or fingers of the guide tube member
during removal and reinstallation of the top tie plate and during intermediate storage
of a guide tube member without demanding time consuming special measures having to
be taken.
[0004] In order to achieve this aim the invention suggests a method according to the introductory
part of claim 1, which is characterized by the features of the characterizing part
of claim 1.
[0005] A guiding pin for carrying out the method according to the invention is characterized
by the features of claim 2.
[0006] The invention will now be described in greater detail with reference to the accompanying
drawings showing in
Figures 1-5 a guide tube member in partial vertical section through the centre axis
of a substantially cylindrical through-hole in a top tie plate included in a nuclear
fuel assembly designed for a pressurized water reactor, whereby the different figures
show in numerical order for one and the same guide tube member the consecutive stages
during the removal and reinstallation of the top tie plate;
Figure 6 in vertical section along line VI-VI in Figure 7 an upper end portion of
the same guide tube member prior to removal or after reinstallation of the top tie
plate;
Figure 7 a partial horizontal section along line VII-VII in Figure 6.
[0007] In the drawings, 3 designates a top tie plate in a fuel assembly for a pressurized
water reactor. The top tie plate has a plurality of substantially cylindrical through-holes
4 with a vertical centre axis 1. A guide tube member.2 for a control rod pin is inserted
with an upper end portion into each through-hole 4. The guide tube member 2 has a
vertical centre axis 1' and contains a guide tube 11 with an inner diameter d1, and
a top sleeve 12. The lower top sleeve portion surrounds the upper end portion of the
guide tube 11 and is fixed thereto. In normal condition the top sleeve 12 is inserted
into a through-hole 4 provided in the top tie plate 3. The substantially circular-cylindrical
wall 4' of this through-hole is provided which a horizontal, annular slot 6. The upper
end portion of the top sleeve 12 is provided with six vertical slits 13 being substantially
evenly distributed along the circumference of the sleeve. The sleeve portions which
are located between the slits have a substantially constant wall thickness and form
six resilient fingers 10. Each finger 10 has a circle arc-shaped, radial projection
14 with a radially outwardly facing surface of a convex, double-curved shape and a
radially inwardly facing surface of a concave, double-curved shape. Together, these
projections 14 form a substantially annular sleeve portion 5, divided into annular
segments having a double-curved, convex, radially outward facing surfaced and a double-curved,
concave radially inward facing surface. When inserting a guide tube member 2 into
a corresponding through-hole 4, the fingers 10 are first bent resiliently inwardly
and then - when their circle arc-shaped projections arrive at the level of the annular
slot 6 - move back radially outwardly thus causing the above-mentioned, substantially
annular portion 5 of the top sleeve 12 - which portion is divided into annular segments
and has a relatively large outer diameter - to snap into the slot 6. The sleeve portion
5, preferably the entire top sleeve 12, has a substantially constant wall thickness.
The top sleeve 12 is made from a thin-walled, metallic tube material, for example
a tube produced by rolling or drawing.
[0008] For the purpose of retaining the annular sleeve portion 5 of the top sleeve 12 in
the slot 6, a metallic, substantially hollow-cylindrical locking sleeve 7 with a substantially
constant wall thickness is inserted into the top sleeve 12 without any significant
play. The locking sleeve 7 is fixed in the axial direction by means of at least one
bulging or projecting part which is arranged in engagement with the concave side of
the annular sleeve portion 5 of the top sleeve 12. The projecting part of the locking
sleeve 7 may be annular or may consist of at least one wart embossed in the wall of
the locking sleeve 7. In the embodiment shown in Figures 6 and 7, three radially outwardly
directed warts 15 are embossed in the wall of the locking sleeve 7 by means of an
expansion tool which is inserted into the locking sleeve 7 when the latter has been
inserted into the top sleeve 12. The locking sleeve 7 may be made from a material
the ductility of which is either considerably higher than or equal with the ductility
of the material from which the top sleeve 12 is made. In the latter case the locking
sleeve may consist of the same material as the top sleeve 12, with the average wall
thickness of the locking sleeve being chosen preferably smaller than 80 % of the average
wall thickness of the top sleeve 12. When the top tie plate 3 is to be removed - for
example, for the purpose of inserting new fuel rods - the locking sleeve 7 must first
be withdrawn. This involves a destructive deformation of the locking sleeve 7, whereas
the top sleeve 12 remains intact. After the top tie plate 3 has been re-applied, a
new locking sleeve 7 is inserted and expanded. The withdrawal of the locking sleeve
is performed by means of a tool having a plurality of withdrawal hooks and being inserted
into the locking sleeve 7 in such a way that the withdrawal hooks engage with the
lower end surface of the locking sleeve 7.
[0009] In a method according to the invention, the locking sleeve 7 in each one of the guide
tube members 2 is first removed. Thereafter, a corresponding guiding pin 8 is inserted
into each of the holes 4, said guiding pin being oriented in the direction of the
guide tube 11. The guiding pin 8 has a lower circular-cylindrical portion 8', an upper
circular-cylindrical portion 8", and a relatively thin, intermediate circular-cylindrical
portion 8"', which is positioned immediately adjacent to the upper portion 8". The
portion 8"'has an axial length which is preferably longer than the thickness H of
the top tie plate 3 and shorter than 3H. Further, the guiding pin 8 is formed with
a circular-cylindrical flange 16, located immediately below the relatively thin portion
8"'. The portions 8", 8' and the flange 16 are dimensioned with such maximum diameters
as to be able to be inserted with slip fit into the hole 4, into the guide tube 11,
and into the top sleeve 12, respectively. A circular-cylindrical part 17 of the guiding
pin 8, located between the flange 16 and the lower portion 8', has a diameter which
is somewhat smaller than the maximum diameter of the lower portion 8' but larger than
the diameter of the relatively thin portion 8"'.
[0010] The upper portion 8" has a horizontal annular end surface 18, in which an annular,
downwardly-facing slot 9 is provided. The radially inner wall of this slot 9 consists
of an extension of the circular-cylindrical surface of the relatively thin portion
8"'.
[0011] During the above-mentioned insertion of a guiding pin 8 into each one of the through-holes
4 (see Figures 1-5), the downwardly-facing annular surface 18 of each guiding pin
is first brought into contact with the upper end surfaces of the fingers
.10. Thereafter, the top tie plate 3 is lifted. This movement causes the fingers 10
of each guide tube member 2 to be bent resiliently radially inwards by the fact that
the annular protrusion 5 is forced out of the slot 6. When the contact between the
surface 18 and the end surfaces of the fingers 10 is broken, the guiding pin 8 falls
down, and the end portions of the fingers 10 slip into the annular slot 9 (Figure
2). The fingers 10 maintain this position in relation to the slot 9 during the continued
lifting of the top tie plate 3 to a level above the upper ends of the guiding pins
8, and also after the top tie plate 3 has been removed as well as during subsequent
corrective maintenance - for example, exchange of fuel rods - and as well during the
reinstallation of the top tie plate 3. Thus the delicate slitted part of each of the
top sleeves 12 is well protected against contact with the lifting devices used during
the repair work and the components handled by said lifting devices. During reinstallation,
the top tie plate 3 is lowered down towards all the guide tube members 2 while at
the same time these members are to be guided into a respective through-hole 4. This
is done without difficulty since each guiding pin 8 - above the slot 9 - tapers in
the direction towards its upper end. When the top tie plate during its reinstallation
has been brought approximately into its normal position in relation to the guide tube
members 2 - as shown in Figure 4 - all the guiding pins 8 are removed, whereafter
the radially outwardly protruding sleeve portion 5 of each guide tube member 2 snaps
into a corresponding slot 6. Finally, a new locking sleeve 7 is inserted into each
one of the guide tube members 2 and is fixed in the same way as the original one.
1. A method for protecting the guide tube members (2) of a nuclear fuel assembly during
removing and/or reinstalling of the top tie plate of the fuel assembly and/or during
intermediate stages, said guide tube members (2) having a vertical centre axis (1)
and a circular cross-section, and being each inserted, with a slitted upper end portion
(2'), in a withdrawable manner into a respective, substantially circular-cylindrical
through-hole (4) in said top tie plate (3), said slitted end portion having an annular
protrusion (5) adapted to snap into an annular slot (6) provided in the wall of said
through-hole (4), said end portion being locked in said snapped position by means
of a locking sleeve (7) inserted in the end portion, characterized in that for removing
the top tie plate from the guide tube members the locking sleeves (7)are first removed,
that thereafter a guiding pin (8) oriented in the direction of the guide tube (11)
is inserted from above into each one of said through-holes (4), said guiding pin being
formed with a lower portion (8'), an upper portion (8"), and an intermediate, relatively
thin portion (8"') located immediately adjacent to said upper portion, said upper
portion (8") and said lower portion (8') being dimensioned to be inserted with slip
fit into said through-hole (4) and into said guide tube member (2), respectively,
said intermediate portion (8"'), at least in the vicinity of said upper portion, having
a diameter that is by so much smaller than the inner guide tube diameter (d1) as to
allow the adjacent upper portion (8") to be formed with an annular, downwardly facing
surface (18) in which an annular, downwardly facing slot (9) is provided large enough
to allow the insertion of said slitted upper end portion 2', that the insertion of
the guiding pin (8) from above into said through-hole (4) is blocked by contact made
between said downwardly facing surface (18) of the guiding pin (8) and the upwardly
facing surfaces of a plurality of axially directed fingers (10) formed by said slitted
end portion of the guide tube member (2) located in the through-hole, that thereafter
said top tie plate (3) is lifted and said fingers (10) - because said annular protrusion
(5) leaves said annular slot (6) - are bent radially inwardly and are inserted with
their ends into said annular, downwardly facing slot (9) and are held in position
in said slot during the continued removal of the top tie plate (3) in relation to
said guide tube member (2).
2. A method according to claim 1, characterized in that, after the top tie plate has
been removed, the guiding pins are further held in their position relative to their
corresponding guide tube members (2) until the reinstallation of the top tie plate
and up to the point where a normal relation between the top tie plate (3) and the
guide tube members (2) is restored, that the guiding pins (8) are then removed, and
that each one of said slitted guide tube portions (2') is again provided with a corresponding
locking sleeve (7).
3. A guiding pin for carrying out the method according to claim 1 or 2, characterized
in being formed with a lower portion (8'), an upper portion (8"), and an intermediate,
relatively thin portion (8"') located immediately adjacent to said upper portion,
said upper portion (8") and said lower portion (8') being dimensioned to be inserted
with slip fit into said through-hole (4) of the top tie plate and into said guide
tube member (2), respectively, said intermediate portion (8"'), at least in the vicinity
of said upper portion, having a diameter that is by so much smaller than the inner
guide tube diameter (d1) as to allow the adjacent upper portion (8") to be formed
with an annular, downwardly facing surface (18) in which an annular, downwardly facing
slot (9) is provided large enough to allow the insertion the slitted upper end portion
2' of the guide tube member.